Starch pastes or gelatinized starches are generally produced by heating aqueous starch suspensions, i.e., those containing sufficient water excess, at temperatures above the so-called gelatinization temperature. This gelatinization temperature is dependent on the type of starch, and is generally markedly above 50.degree. C. The starch granules which occur as discrete partially crystalline particles swell intensely in this process. While some of the starch polymers (in particular the amylose) go into the solution, the insoluble, swollen, amylopectin-rich particles produce a matrix, which can no longer be separated into individual particles by conventional techniques. Accordingly, the physical properties of the starch pastes or gels thus obtained are determined by this matrix of dissolved amylose and swollen particles.
During the drying, and in particular during the roller drying which is preferably used for the production of gelatinized starches, the structure present in the pastes is further destroyed. In the gelatinized starches thus obtained, the granular structure is completely destroyed at the latest by the grinding of the film taken off from the roller. However, gelatinized starches produced in this manner are cold-swelling, in contrast to native starches.
In the light of this background, two processes have been developed, by means of which destruction of the granule shape and size caused by the swelling can be prevented, but nevertheless pregelatinization, i.e., cold swelling properties, of the starch granules can be achieved. The starches thus obtained are called "granular cold-water-swelling starches" (GCWS starches) and have special properties because of their granular character.
These granular cold-water-swelling starches are either produced by pregelatinization during spray-drying or by heating in alcohol-water mixtures. In the first process, the drying process follows the pregelatinization process so rapidly that swelling and aggregation of the granules is prevented. In the alcohol-water mixtures, the water content is sufficient for a complete gelatinization, but, at the same time, the alcohol content is high enough to prevent swelling.
A particular property of such GCWS starches is, for example, that very smooth pastes are retained therefrom, a property which is of interest particularly in the production of low-fat systems, i.e., in systems in which fat is fully or partially replaced by starch. It is thought that the granular character of these starches, which simulates fat droplets, is essential for this application.
Granular starch pastes and gels containing granular pregelatinized starch granules today cannot be produced directly, but can be produced only by the processes described above, in which they are obtained as pulverulent dry products. In this context, the above mentioned swelling which begins immediately in the presence of excess of water is, of course, one reason, but not the principal reason, why it is possible to produce GCWS-starches only by these specific processes. An even more critical fact is that the individual starch granules present in a starch suspension gelatinize, i.e., swell, at different temperatures. Usually, the last granules of a suspension only gelatinize at a temperature about 10.degree. C. above that of the beginning of gelatinization. This means that a uniform gelatinization can never be achieved, not even with an extremely precise temperature control.
Pastes of native starches produced in the conventional manner by heating in excess of liquid are, depending on the type of starch, clear (e.g., potato starch or waxy maize starch) or are opaque to a greater or lesser extent (e.g., maize starch), but, in comparison with a finely dispersed emulsion containing fat droplets, they are always transparent. In order to obtain a higher "whiteness power" in the case of starch-based fat substitutes, a number of processes have been proposed. Products obtained by these processes can be classified as so-called "microparticulates," i.e., discrete particles of approximately the size of the fat droplets present in emulsions.
A number of similar products are described in the literature and patent literature. These products share the property that the crystalline parts of the starch granule were enriched. Retention of a certain crystallinity is therefore essential for (starch-based) microparticulates. (In addition, protein- and cellulose-based products of this type have also been developed.)
However, because of the particular properties of pastes containing pregelatinized, restricted swollen starch granules (and furthermore still partially crystalline), there is great interest in producing pastes of this type directly, and not via spray-drying or via an alcohol-water treatment.
Surprisingly, it has now been found that this can be achieved. Moreover, pastes of this type can be produced directly in a very simple manner, by using high pressures.
The high-pressure treatment of starch is known in principle.
In Carbohydrate Research 93: 304-307 (1981), the effect of high pressure on the gelatinization temperature of starch has been investigated. Up to about 1500 atm, the gelatinization temperature is displaced upwards by increase in pressure by 3-5.degree. C./100 atm. Pressures higher than 2500 atm could not be investigated in this study.
In Carbohydrate Polymers 2: 61-74 (1982), microscopic and DSC (differential scanning calorimetry) studies report the effect of pressure in the range from 200-1500 MPa on the gelatinization of wheat starch and potato starch. In contrast to the publication above, the hydrostatic pressure is said to decrease the gelatinization temperature. Except for the specimens containing little water, there is said to be no great effect of pressure on the product properties (staining, behavior under the polarization microscope, later gelatinization at atmospheric pressure).
This latter finding is confirmed in Carbohydrate Polymers 2: 91-102 (1982), according to which a slight increase in the gelatinization temperature was observed only at the beginning, which then remained constant in the pressure range 150-250 MPa, and which decreased slightly at higher pressures up to 400 MPa.
Further studies, such as those described in Starke 45: 19-24 (1993), carried out in a DSC apparatus with barley starch and potato starch at low water contents (10-34%) and high temperatures (50-250.degree. C.), show that, under these conditions, higher enthalpy values are required for gelatinizing the starch.
From these partly contradictory literature data, which furthermore are only based on studies carried out with microquantities, few conclusions can be drawn for the use of high-pressure-treated starches in industrial practice.